1. Field of the Invention
The present invention relates to a one-terminal type level control circuit for preventing an input signal from a sensor to a signal processing circuit, such as a microprocessor or the like, from becoming excessively large.
2. Description of the Prior Art
It is well-known in the art to employ in an automotive vehicle or the like a system that controls respective parts, such as an engine, in accordance with an output signal from various sensors, such as a temperature sensor, a pressure sensor and so forth. For instance, as shown in FIG. 1, ON-OFF or analog output signals are applied, via resistors R21 to R2n, to a microprocessor .mu.CPU of centralized control equipment CONT from sensors S1 to Sn in accordance with temperature, pressure and so forth. A voltage V.sub.B of a battery B mounted on a vehicle is converted, by a stabilized power source circuit PW composed of a DC-DC converter and so on, into a power source voltage V.sub.CC of the microprocessor .mu.CPU, for example 5 V, which is provided to the sensors S1 to Sn via resistors R11 to R1n, respectively. Capacitors C1 to Cn and Ca1 to Can are provided for removing noise. Reference characters D11 to D1n and D21 to D2n indicate diodes.
The sensors S1 to Sn and the centralized control equipment CONT are usually located at remote positions and, accordingly, their grounding points are provided at different positions on the body of the vehicle. Consequently, a potential difference may sometimes occur between the grounding points with respect to their grounding resistance values of the vehicle body. For instance, in the case where the potentials at the grounding points of the sensors S1 to Sn have risen higher than the potential at the grounding point of the centralized control equipment CONT, the microprocessor .mu.CPU may be supplied with sensor output signals of levels higher than a predetermined value. The diodes D11 to D1n and D21 to D2n, and the capacitors C1 to Cn, are provided to hold the input level at the predetermined value in such a case, protecting the microprocessor .mu.CPU.
In the prior art example, since the power source of the microprocessor .mu.CPU is used as a power source for the sensors S1 to Sn, the capacitance of the stabilized power source circuit PW must be made large, and a sensor having an independent power source cannot be connected directly to the centralized control equipment. Furthermore, the noise preventing effect of the capacitors C1 to Cn is small. In the case where they are connected to the side of the input terminal of the microprocessor .mu.CPU, as is the case with the capacitors Cal to Can shown, the noise preventing effect is large, but the microprocessor .mu.CPU formed as an integrated circuit may be damaged by charges stored in the capacitors Cal to Can.